Rotational Molding in Structures

Posted by Michael Paloian on 30 June 2010

 

Applications for rotational molding are being discovered everyday as molders, end users and designers creatively apply the benefits of the process to ever more challenging markets. The trend for these new applications is moving toward more demanding load bearing structures which are subjected to a wide range of environmental conditions. These more complex applications require careful considerations of load distribution and stress concentrations based on overall structural integrity. Consideration of environmental factors such as temperature, ultraviolet radiation and chemical exposure are critical to assure long term product performance in outdoor applications. Combinations of stress, temperature, and material degradation over extended periods of time present designers with an extremely challenging set of factors to analyze when designing rotationally molded products for the long term.

 

Unfortunately data for commodity materials like polyethylene which is the primary material of choice for rotational molding, is severely limited. Commercially available data pertaining to the affects of UV on impact strength or tensile strength is non-existent for polyethylene. Material degradation caused by extended exposure to common harsh chemicals is also not commercially available. Most importantly, there is little to no data available for the affects of temperature on tensile strength or modulus. Some material suppliers like Exxon have taken steps to develop more useful engineering data for their polyethylene resins. During this year’s SPE ANTEC conference I was recently informed by an Exxon marketing manager that they have recently generated more comprehensive stress-strain data for some of their resins. Although this is a step in the right direction, they have limited the distribution of this information to their molder customers because they believe the information is too valuable to hand out to the general public. I was very surprised and amused by this short sighted thinking since most molders don’t care about stress strain curves and wouldn’t know what to do with them anyway. Engineers and designers specify resins. These are the individuals who will indirectly benefit resin manufacturers willing to invest time and money generating such data.

 

 

Generating material data is expensive, time consuming and potentially risky for material suppliers. However, such information is readily available to the general public for so called engineering resins like polycarbonate, acetal, nylons, PBT and dozens of others. These resins command premium prices and generate higher profits for the suppliers. They are specified by engineers specifically because they can be verified to comply with performance requirements stipulated in product specifications based on available published performance data.

 

Virtually every rotational molding application could either be improved or optimized by matching material properties to specific performance expectations. The challenges for designers and engineers are to identify the critical environmental factors affecting product performance. This information must then be properly analyzed based on a specific design and material, then verified to comply product specifications. Today’s CAD and computer technology provide designers with the tools to perform sophisticated structural analyses that could only be simulated by university PHD’s a few years ago. However, the age old adage “garbage in, garbage out” still applies. The results are only as good as the information and assumptions.

 

If the rotational molding industry is to make any significant improvements within established markets like tanks, pallets, or kayaks, or penetrate new markets in transportation, underground water structures etc, everyone in the industry must elevate the standards to higher levels. Designers must continually improve their skills and expand their knowledge base to deliver more sophisticated products. Material suppliers must provide more a more diverse resin portfolio with reliable and meaningful data which will ultimately expand their markets and sales. Manufacturers should improve their quality productivity and services to help the industry continue to expand into new untapped markets.

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